The present invention generally relates to the field or technique for checking the trim of the wheels of motor vehicles and more particularly it relates to a universal self-centering device for supporting heads or sensors for checking wheel trim and acquiring data related thereto.
It is known that said trim is checked by using adapted heads or sensors for data acquisition detailed description whereof is unnecessary.
It is also known that two procedures can be followed for this checking operation: one uses four heads (two for the front axle and two for the rear axle), while the other one uses two heads for the front axle and two brackets for the rear axle.
The brackets are not described in detail, since they are not a characterizing part of the invention, while the head suspension device will be described further hereinafter.
In any case, adapted data acquisition and transmission means are associated with the heads and are placed under the control of a central unit for processing and displaying the data acquired by the heads.
Self-centering devices are known for associating one of the heads with a wheel of a wheel assembly (wheel+tire) of a motor vehicle; these devices comprise a plurality of identical grip elements, usually termed claws. The claws are usually of the double-acting type, i.e., they are adapted to engage the bead retaining rim of a wheel respectively from the inside (typically for sheet metal wheels) and from the outside (typically for alloy wheels).
Some of these conventional devices have an even number of claws, typically four, in which the claws are mutually connected in pairs so as to form a monolithic unit and thus move together, while in other conventional devices the claws, typically three, are distinct and separate.
The invention relates to devices of the second kind, particularly to those that comprise a contoured plate which is typically shaped like a three-pointed star and is provided with a central hub, which protrudes in order to suspend a data acquisition head, and with a set of three identical slots whose longitudinal axes intersect the longitudinal axis of said hub.
A claw is slidingly associated with each individual slot, at the opposite end with respect to the one occupied by the hub, and is provided with a respective locking and release clamp.
Moreover, these devices are provided with a tire grip unit, which is particularly convenient when the claws engage from the inside the bead retaining rim of a wheel. This is done, as is known to those skilled in the art, in order to keep the free ends of the claws in contact with the side of the bead retaining rim and thus allow the corresponding head to correctly acquire the data of the wheel assembly to be checked even when the outer peripheral margin of the bead retaining rim is damaged.
In particular, the tire grip unit comprises two contoured and adjustable-length arms which are substantially shaped like an L or a pawl and are articulated to the plate along two parallel axes arranged on either side of the hub.
The free end of the arms is shaped like a pointed tooth or wedge and the arms can oscillate between an inactive position, in which they are provided at the side of the plate that is occupied by the hub, and an active position, in which they are at the opposite side. There is also provided a threaded system, by means of which the free ends can be moved mutually closer and further apart in order to engage them with the tire and disengage them therefrom.
The extensive use of such conventional devices has pointed out the following problems.
A first problem arises from the fact that all the claws must be moved and locked individually every time one works on a wheel having a different diameter, and this is inconvenient for the following reasons.
First of all, the positioning and locking of the individual claws entails relatively long and troublesome operations; secondly, this way of working can lead to inaccurate mounting of the device.
This occurs because a relatively inaccurate positioning of the claws of a head supporting device, mounted for example on the rear right wheel assembly of a motor vehicle, may entail, as has indeed been observed, an excessive vertical misalignment of its head with respect to the head of the device that is mounted, for example, on the right front wheel assembly of the motor vehicle.
Since the central data processing and display unit can work only if the vertical misalignment is within a preset allowable range, when the misalignment is out of this range the central unit does not operate, indeed because of the inaccurate mounting of at least one head supporting device; accordingly, the operator must check the entire system and dimount and remount said at least one incorrectly mounted device.
Other problems of these conventional devices arise from the direct articulation to the contoured plate of the curved arms that constitute said tire grip unit.
Owing to this direct articulation:
first of all, the connection of the curved arms to the respective threaded actuation system is constructively complicated and therefore also expensive; PA1 secondly, the presence of the articulations forces to arrange the three points of the star-shaped plate with an irregular angular distribution, i.e., so that two points are spaced for example by 90.degree. from each other and by 135.degree. from the third point, whereas an optimum grip of the claws on the wheel occurs when the three points are angularly equidistant.
Conventional devices have also proved to be unsatisfactory, owing to the fact that the pointed ends of the contoured arms tend to cut into the tire and thus damage it, particularly when the arms are inadvertently fastened with a significant force, as occurs rather often since the screw-and-nut kinematic actuation system does not provide the operator with the sensitivity required to adjust the fastening.
Finally, the conventional devices are inadequate from the point of view of operating flexibility, owing to the fact that they are unable to utilize the centering holes that are provided in the central part of the special wheels currently mounted on high-performance motor vehicles and have the purpose of correctly positioning units or accessories for checking the characteristics of the respective wheel assembly and/or the trim of the corresponding motor vehicle.
Accordingly, in such cases it is possible to achieve optimum control of the wheel assembly and/or of the trim only by means of specific equipment.